One of the more important features in a cockpit is the Heads Up Display (“HUD”) whereby flight control information is projected onto the wind screen of the aircraft so that the pilot may receive the information without taking his eyes from the airspace in front of him. However, there has been a growing interest in moving away from HUD systems to NTE or to NTE-HUD systems with head mounted viewing and sensor components that may be attached to a pilot's earphones or to his helmet.
A NTE system is characterized by a small display screen that is suspended directly in front of one or both of the pilot's eyes such that the displayed virtual object or image moves about the display screen as the pilot turns his head to look for other aircraft, look at his other controls and instrumentation, etc. The NTE display is otherwise transparent such that the pilot may see through or see past the display.
It would be disconcerting, disorientating and annoying to the pilot if information being sent to the NTE display was constantly visible in his display as he looks around him for other aircraft or for cockpit instrumentation. As such, a NTE display processor is programmed to register or be conformal with the NTE display information within a specific area within the cockpit such that when the pilot is looking at the area of image registration the NTE information is visible in his NTE display and when he turns, nods or cocks his head (i.e. yaw, pitch and roll), the NTE information moves in the opposite direction, and even out of view, until the pilot returns his head to a normal flight position.
The image registration is typically established by attaching one or more markers or marker bars to physical locations in the cockpit. The markers may have a particular shape or pattern, or may emit light at a particular frequency. The shape, pattern or emission frequency can be detected by the pilot's NTE headset and the detected position of the marker then causes the NTE processor to render the display on the NTE display screen only when the pilot's head is in a desired viewing position range relative to the marker.
However, the registration process as practiced in the art has not been perfect. It has been noticed that the majority of head movements are side-to-side (yaw) movements and the speed of the yaw movement has been determined to be in the 1000 degrees per second range compared to a nodding action (pitch) and cocking action (roll), which have been measured to be typically in the 400 degrees per second range. Pitch and roll movements are therefore of lesser concern.
Because of the relatively rapid side-to-side (yaw) speed, blurring and/or swimming of the NTE image or virtual object can occur when a pilot turns his head. Blurring is the loss of contrast due to multiple overlapping image rendering. Swimming is the phenomenon that occurs when different parts of an image move at different speeds.
Both blurring and swimming tend to occur because NTE head positioning sensor only periodically updates the position of a pilot's head. If a rapid yaw motion should occur during the interstitial time period between head position updates, sequential information frames will be presented with an abnormally large physical separation. The size of this separation which is a function of the head's travel time and velocity, and results in distortion such as horizontal blurring. The longer the latency between head position measurements the worse the distortion.
However, even in the absence of head sensor delays, some blurring and swimming will continue to occur due to the NTE display's inbuilt horizontal raster scan periodicities. Typical visual distortions due to horizontal raster scan periodicities include a blurring effect and/or a tilting of the virtual object image. Therefore, there is a need to improve an NTE display to minimize the distortion resulting from a pilot's head movement.